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The effective management of water resources faces unprecedented pressures from growing demand and shifting climate among other stressors. Widespread issues of both water quantity and water quality affect many regions. One common problem facing many areas is the excess supply of nitrate (NO3) to receiving bodies such as aquifers, rivers, streams, and estuaries. Excess nitrate is linked to human activity and can have detrimental effects of human and ecosystem health. Addressing these issues and ensuring that humans and ecosystems have sufficient supplies of clean water requires a more complete understanding of the complex linkages between water quantity and quality across space and time. In this dissertation, I present four original studies across a range of scales investigating the flow of water across and through landscapes that have been altered by human activity. In each study I ask the fundamental question how does the magnitude, timing, and/or rate of water flow affect key water quality parameters? Chapters One, Two, and Three focus on investigating water quality improvements during managed aquifer recharge (MAR), a technique for improving groundwater supply and quality through targeted infiltration. In Chapter One, measurements of water quality parameters during controlled percolation experiments revealed that soil amendments could enhance nitrate removal during infiltration. In Chapter Two, column studies conducted on soil cores collected from MAR sites demonstrated that the rate of infiltration and the presence of a soil amendment were key factors controlling nitrate removal and affecting microbial community composition during MAR. In Chapter 3, we synthesized data from various scales to develop a novel, spatial model of nitrate removal during MAR based on soil and fluid properties. Finally, in Chapter Four, similar concepts are applied to the analysis of nitrate mobilization in streams and rivers of agricultural watersheds to analyze the patterns and timing

The effective management of water resources faces unprecedented pressures from growing demand and shifting climate among other stressors. Widespread issues of both water quantity and water quality affect many regions. One common problem facing many areas is the excess supply of nitrate (NO3) to receiving bodies such as aquifers, rivers, streams, and estuaries. Excess nitrate is linked to human activity and can have detrimental effects of human and ecosystem health. Addressing these issues and ensuring that humans and ecosystems have sufficient supplies of clean water requires a more complete understanding of the complex linkages between water quantity and quality across space and time. In this dissertation, I present four original studies across a range of scales investigating the flow of water across and through landscapes that have been altered by human activity. In each study I ask the fundamental question how does the magnitude, timing, and/or rate of water flow affect key water quality parameters? Chapters One, Two, and Three focus on investigating water quality improvements during managed aquifer recharge (MAR), a technique for improving groundwater supply and quality through targeted infiltration. In Chapter One, measurements of water quality parameters during controlled percolation experiments revealed that soil amendments could enhance nitrate removal during infiltration. In Chapter Two, column studies conducted on soil cores collected from MAR sites demonstrated that the rate of infiltration and the presence of a soil amendment were key factors controlling nitrate removal and affecting microbial community composition during MAR. In Chapter 3, we synthesized data from various scales to develop a novel, spatial model of nitrate removal during MAR based on soil and fluid properties. Finally, in Chapter Four, similar concepts are applied to the analysis of nitrate mobilization in streams and rivers of agricultural watersheds to analyze the patterns and timing